Is it valid for an Imam to remember another Surah during recitation, interrupt the recitation, restart Surat Al-Fatihah, and then complete the prayer?

The primary and preferable act for the worshiper is to recite a complete surah after Al-Fatihah. However, the basic recommendation is fulfilled by reciting any portion of the Quran. There is no objection to reciting throughout the prayer until the worshiper completes a ختمة (full recitation of the Quran). It is not appropriate to deliberately switch from one surah to another during prayer without a legitimate শরعي ( Shar'i) purpose. Similarly, intentionally repeating Al-Fatihah is either forbidden (haram) or disliked (makruh), though it does not invalidate the prayer according to the preponderant opinion.